China’s Defence Ministry said on Saturday it had been in talks with the United States about returning an underwater drone taken by a Chinese naval vessel in the South China Sea, but the U.S. was not helping by “hyping up” the issue.

The drone was taken on Thursday, the first seizure of its kind in recent memory, about 50 nautical miles northwest of Subic Bay off the Philippines, just as the USNS Bowditch was about to retrieve the unmanned underwater vehicle (UUV), U.S. officials said.

The Defence Ministry said a Chinese naval vessel discovered a piece of “unidentified equipment” and checked it to prevent any navigational safety issues, before discovering it was a U.S. drone.

“China decided to return it to the U.S. side in an appropriate manner, and China and the U.S. have all along been in communication about it,” the ministry said on its website.

“During this process, the U.S. side’s unilateral and open hyping up is inappropriate, and is not beneficial to the smooth resolution of this issue. We express regret at this,” it added.